1. Field of the Invention
The present invention relates to a sheet aligning device that performs a sheet-aligning function, a sheet processing device that includes the sheet aligning device, and an image forming apparatus, such as a copier, printer, facsimile, or a digital multifunction peripheral (MFP) that performs copying, printing, and facsimile functions, that includes the sheet processing device.
2. Description of the Related Art
The prevalence of sheet processing devices equipped with functions such as sheet aligning, finishing, and folding processing has increased in recent years parallel with the prevalence of image forming apparatuses. For example, Japanese Patent Application Laid-open No. 2005-132616 discloses a sheet post-processing device that receives sheets discharged from an image forming apparatus one by one and performs a sheet post-process on each sheet, and that includes a processing tray that includes an aligning unit that aligns sheets, a stapling unit that staples a sheet bunch, and a sheet ejecting unit that ejects the stapled sheet bunch in a sheet discharge direction, a first conveying path, which is the main conveying path, that includes a detecting unit that detects the sheet, and a conveying unit that conveys the sheet in a downstream direction or in a reverse direction, a second conveying path that receives the sheet conveyed in the reverse direction and conveys the sheet to a designated position and is capable of retaining a plurality of such sheets, a switching unit that switches the conveying path of the sheet from the first conveying path to the second conveying path, and a catch tray that receives the sheet discharged by a sheet discharging unit. In the sheet post-processing device, a plurality of sheets belonging to one sheet bunch is conveyed for sheet post-processing in such a way that the sheet conveyed by the second conveying path is stacked with the sheet conveyed by the first conveying path when being discharged to the processing tray. In a mixed loading mode for sheets of varying sizes form a single sheet bunch, a preceding sheet is retained if it is of retainable size. In this disclosed invention, the second conveying path is employed to secure a processing time/expelling time for the sheet or the sheet bunch.
However, generally, in an image-forming apparatus equipped with a sheet post-processing device, when the sheet already received is being subjected to post-processing such as aligning and stapling, the processing tray is unable to receive the succeeding sheet, necessitating increased gap between the sheets and resulting in a drop in productivity.
Mechanisms, such as that mentioned in the above invention, for temporarily retaining the sheets prior to being discharged into the processing tray, have been proposed to address the problem. Such a mechanism retains and stacks a first few sheets of the next sheet bunch before discharging them to the processing tray, thus avoiding discharge of single sheets when the previous sheet bunch is being processed, and a drop in productivity. However, such a mechanism is often complicated and tends to increase the bulk as well as the cost of the image forming apparatus. Further, this processing method is ineffective in preventing a drop in productivity if there is only a single sheet in each lot.